The effect of vacuum desalination on the underwater glazed pottery cultural relics from the “Nanhai Ⅰ” shipwreck

The glazed pottery cultural relics from the “Nanhai Ⅰ” shipwreck were desalinated using deionized water as the desalting solvent in a self-designed vacuum desalination device. The desalination effects of the vacuum method and the flowing water method were compared through ion content determination, electrical conductivity and chloride ion concentration measurements, microscopic morphology observations and elemental composition analysis. After desalination for 48 hours, the dechlorination amounts of the vacuum method and the flowing water method account for 78.94% and 41.52% of the total dechlorination amounts, respectively. When the desalination endpoint was reached, the vacuum method removed a total of 44.66 mg of chloride ions, corresponding to a desalination rate of 1.31 mg/d. In contrast, the flowing water method removed a total of 20.11 mg of chloride ions, resulting in a lower desalination rate of 0.65 mg/d. The results confirm that the vacuum desalination method has a higher desalination rate. After vacuum desalination, the salt in the glazed pottery has been essentially completely removed, and no obvious NaCl crystals were observed. However, after flowing water desalination, a small amount of NaCl crystals were still found remaining in the glaze layer cross-section, indicating that the vacuum desalination method is superior to the flowing water method.